The present invention relates to a miniature synchronous motor, as used, for example, in synchronous watches or as a stepping motor in step-by-step switches, time switches and program switches. In many applications for example, in the case of synchronous watches a particularly silent operation of the synchronous motors is required. Moreover, a high performance is frequently also required which, for example, in miniature synchronous motors is attained only with a group of main and auxiliary poles on the stator, which groups are substantially oppositely positioned. Of these motors it is also known that radial vibrations may occur which, in rotor bearings with no play, may lead to noises.
It is known to provide a pawl with slight pressure abutment against a disc of the rotor to determine the direction of starting of a synchronous motor and to absorb rotary vibrations of the rotor. See, U.S. Pat. No. 3,496,393. It will thereby act with a minimum of compressive pressure against a disc of large circumference, in which case it is possible to produce a sufficient transverse pressure in the motor bearings to silence noises. However, the slight radial application pressure in known motors affects substantially only one bearing and for this reason it could not substantially contribute towards any silencing.
The object of the present invention is to provide a particularly silent miniature synchronous motor.
According to the present invention there is provided a miniature synchronous motor having a damping body acting on the rotor, comprising a non-rotating damping body mounted freely on the rotor shaft and which is urged in an at least a substantially radial direction against the shaft.
Preferably a damping body made of plastic material may be urged by a wire spring or a spring integrally formed therewith against the rotor shaft. It has been found that this measure permits a substantial silencing or absorption of noise, because by this concept a favorable ratio of friction moment relative to radial force is obtained, i.e. the relatively large radial force for suppressing radial rotor vibrations only has a small friction moment, since the rotor shaft is generally made of ground and polished steel and has a small diameter. This also makes it possible for plastic material bearings to be used which, as such, have greater tolerances and hence more bearing clearance than metal bearings, so that the plastic material bearings compared with metal bearings also contribute to the reduction of noise. Moreover, this device also damps rotary vibrations and the double rotary field caused by unavoidable tolerance deviations of the stator poles, rotor poles and the air gap, and also starting up in the wrong direction is substantially prevented.
To obtain an optimal silencing the application pressure or damping force should be at least equal to the rotor weight, so that, for optional installation positions of the motor, the bearing play is cancelled. This force is preferably at least equal to the vector sum of the rotor weight and the magnetic radial forces acting on the rotor. Moreover, the direction of the application pressure force should be such that it acts substantially to an equal extent on the two motor bearings.
The invention will now be described further, by way of example, with reference to the accompanying drawings.